avcodec/internal: Move ff_get_buffer() to decode.h

Only used by decoders (encoders have ff_encode_alloc_frame()).

Also clean up the other headers a bit while removing now redundant
internal.h inclusions.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/alsdec.c

@@ -34,6 +34,7 @@
 #include "bgmc.h"
 #include "bswapdsp.h"
 #include "codec_internal.h"
+#include "decode.h"
 #include "internal.h"
 #include "mlz.h"
 #include "libavutil/samplefmt.h"